The present invention relates generally to motor systems and, more particularly, to a system and method for controlling a motor over a dynamic operating range, specifically, changes between torque control and speed control.
Motors and linked loads are one type of common inductive load employed at many commercial facilities. To drive a motor, an inverter formed from a plurality of switches is controlled to link and unlink positive and negative DC buses to motor supply lines. The linking-unlinking sequence causes voltage pulses on the motor supply lines that define alternating voltage waveforms of controlled magnitude and frequency. When controlled correctly, the waveforms cooperate to generate a rotating magnetic field inside a motor stator core. In an induction motor, the magnetic field induces a field in motor rotor windings. The rotor field is attracted to the rotating stator field and; thus, the rotor rotates within the stator core. In a permanent magnet motor, one or more magnets on the rotor are attracted to the rotating magnetic field.
The inverter and control circuitry are collectively referred to as a motor drive unit. By controlling operation of these components, the motor drive unit controls the overall operation of the motor. A variety of control methods are commonly employed to control the operation of the motor. For example, two common control methods are called speed control and torque control. As suggested by the name, speed control methods seek to control the overall operation of the motor by using the speed of the motor as the control criteria. Likewise, torque control methods use the torque experienced by the motor as the primary control criteria for controlling the motor.
The particular control method employed by a motor drive unit is typically dependent upon the load associated with the motor and/or the operation/application being driven by the motor. For example, motors are commonly employed in the paper manufacturing and printing industries to move the web through various processing stages. In this case, an electronic line shaft is often employed to move the web or paper material over rollers and through various stages of the printing process. Within such applications, the motors are typically torque controlled to ensure that a proper and consistent tension is applied to the web as it is moved to each stage.
However, in most applications, there are instances when it is advantageous to switch between various control methods. For instance, with respect to the example of web fabrication or printing applications that are typically torque controlled, it is often necessary to switch from torque control to speed control or vice versa, for example, when the end of the web is reached or when a line break is experienced. That is, when the resistance presented by the web is removed, such as when the end of the web is reached or a line break is experienced, the control method is typically switched to speed control in order to avoid excessive speeds. However, since the need to change from torque control to speed control (or vice versa) is often sudden and unexpected, a motor drive unit may remain in torque control mode too long, which can result in driving the motor to an excessive over-speed that could potentially damage the associated load or product.
As a result, a variety of control algorithms, such as torque reference control methods, have been developed that seek to effectively and efficiently transition between torque and speed control methods. One such method referred to as speed limited adjustable torque (SLAT) control relies upon a min/max comparison of a torque reference to a speed regulator output to select the torque reference for the drive.
Referring to FIG. 1, a traditional system for implementing SLAT control includes a min/max torque reference control system 1. The min/max torque reference control system 1 is designed to determine the control method that should be implemented based on the current operating conditions and includes a proportional integral (PI) regulator 2 and min/max selector 3. The input to the PI regulator 2 is a speed error 4 that is calculated by a circuit that determines the difference between an application-dependent speed reference bias 5 and the actual speed of the motor delivered as a motor speed feedback 6. In this regard, the application-dependent speed reference bias 5 serves to limit the actual over- and/or under-speed that the drive unit will permit the motor to achieve should the speed limitation be removed (e.g., a line break). In this regard, the application-dependent speed reference bias 5 causes the PI regulator 2 to integrate to its maximum absolute value limit, which is then delivered as a speed regulator output (SRO) 7 to the min/max select 3.
Hence, one input to the Min/max select 3 is the SRO 7. The other input to the min/max select 3 is an external torque reference (ETR) 8. The min/max select 3 acts as a controller that selects the algebraic or absolute minimum or maximum value of SRO 7 or the ETR 8 and delivers that value as an internal torque reference (ITR) 9 for the drive based on whether it is presently configured to detect an over speed (min mode) or an under speed (max mode) condition.
However, this design is still prone to excessive speed overshoots when switching from torque control to speed control. For example, the level to which the integral term is permitted to integrate can vary significantly between products. In particular, when transitioning to speed mode, the min/max torque reference control system 1 requires that the SRO 7 slew to a value that results in the min/max select 3 selecting the ETR 8. Therefore, the amount of actual over or under speed that occurs before being controlled will vary based on the specifics of the motor drive unit and application-dependent integral time constant of the PI regulator 7.
As a result, some systems have been developed that attempt to implement a “bumpless” transition to speed control. For example, some systems have been designed that reset the integral term of the PI regulator 2 to a level that results in the SRO 7 being equal to the existing ITR 9. While this does reduce the “bumps” experienced when switching to speed control from torque control, if the speed error 4 is particularly high and the integral slew rate is relatively low, the integral term will be reset to a relatively high positive value. However, based on the application requirements and the speed error 4, a high level of negative torque may be required. In this case, due to the relatively long time constant of the integral term, the motor drive unit will cause the motor to deliver this inappropriate and potentially damaging forward torque for an extended period of time.
Therefore, it would be desirable to have a system and method for switching between control methods of a motor drive unit, such as switching between torque control and speed control, that is less prone to variations between motor drive units, motors, applications, and the like so as to provide smooth and accurate transitions between control methods.